Recording medium having substrate with thickness dependent on numerical aperture of object lens, method of forming the optical medium and optical recording/reproducing apparatus

ABSTRACT

A high density optical disc that can be driven changeably with the existent optical disc by the same driving apparatus. On the recording surface of the optical disc, a light transmissive layer with a thickness of about 0.2 to 0.4 mm is formed. The recording face is accessed by allowing a light beam with a wavelength of 395 to 425 nm in a spot shape to be irradiated onto it. Also, the light beam is converged in a spot shape by an objective lens having the numerical aperture of about 0.62 to 0.68.

This application is a Continuation of application Ser. No. 09/908,807,filed on Jul. 20, 2001 now U.S. Pat. No. 6,747,938, which is aContinuation of application Ser. No. 09/334,894, filed Jun. 17, 1999U.S. Pat. No. 6,345,034 B1 issued Feb. 5, 2002, the entire contents ofwhich are hereby incorporated by reference and for which priority isclaimed under 35 U.S.C. § 120; and this application claims priority ofApplication Nos. P98-23005 and P98-63381 filed in Korea on Jun. 18, 1998and Dec. 31, 1998, respectively, under 35 U.S.C. § 119.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to an optical recording medium for allowing aninformation to be accessed optically, and more particularly to anoptical recording medium that is adaptive for recording information at ahigh density. Also, the present invention is directed to a recording andreproducing apparatus that is suitable for recording and reproducing ahigh density optical recording medium.

2. Description of the Related Art

The quantity of moving picture information, such as movie, has beenreduced sharply in accordance with developments of digital pictureprocessing techniques and moving picture compression techniques. Forinstance, analog video signals for two hours structured by a nationaltelevision system committee (NTSC) system or a Phase alternating by line(PAL) system have an information amount of approximately 80 Gbytes,whereas digital video signals for two hours compressed by “MPEG-2”,which is a moving picture compression standard suggested by the movingpicture expert group, have an information amount of approximately 15Gbytes. As video signals become compressed to have such a smallinformation amount, an optical recording medium, such as compactdisc(CD) or digital versatile disc(DVD), also has been required to storeabout two hours worth of digital video signal information.

Meanwhile, an example of an optical recording medium having the largestrecording capacity includes a DVD. Beam spots irradiated onto signaltracks of the DVD must have such a size that can minimize cross-talkcaused by adjacent signal tracks. To this end, an optical system forrecording and reproducing the DVD includes a semiconductor laser forgenerating a red laser beam with a wavelength of 650 nm, and anobjective lens with a numerical aperture of 0.6. A DVD recorded usingsuch an optical system is not suitable for recording two hours of movingpicture information because only up to 4.7 Gbytes can be recorded.

In order to enlarge a recording capacity, there has been discussed ascheme of using a blue laser beam having a shorter wavelength than a redlaser beam. A blue laser for generating such a blue laser beam will becommercially available soon in accordance with a development of GaNgroup laser. It has been known that this blue laser generates a laserbeam with a wavelength of about 400 nm. An optical pickup including theblue laser can access an optical recording medium for blue laser,hereinafter referred to as “HD(high density)-DVD”, as well as anexistent DVD. To this end, assuming that the HD-DVD have a substrate(i.e., light transmission layer) thickness of 0.6 mm (equal to that ofthe existent DVD) and that an optical pickup having a blue laser(hereinafter referred to as “blue laser pickup”) uses an objective lenswith the numerical aperture of 0.6, because a diameter of beam spotsirradiated onto a disc by the blue laser pickup has a dimensionproportional to (λ/NA)², the HD-DVD has 2.51 times the recording densityof the conventional DVD. In other words, when the HD-DVD is manufacturedin a form identical to the existent DVD, but has a recording capacity of4.7 GB×2.51=11.8 GB.

Further, the blue laser pickup has an average optical aberration amountsmaller than Marchel's criterion of 0.07λ so as to provide beam spotswithin a diffraction limit and an excellent signal-to-noise ratio(S/N).The average optical aberration amount is significant when it is given ina unit of wavelength, and which includes a spherical aberration, a commaaberration, a astigmatism and so forth. It has been known that such anaverage optical aberration amount is influenced by a comma aberrationamount, which is inversely proportional to a wavelength(λ) of the beam.The comma aberration amount appears because of disc tilt, and isproportional to “t·(NA)³/λ”. In other words, the average opticalaberration amount increases in accordance with a tilt amount of thedisc. Accordingly, when the blue laser pickup has a tilt margin of ±0.6,which is equal to the existent optical pickup for DVD, the HD-DVD cannot have the above-mentioned recording capacity. This is caused by theincreased comma aberration caused by the decreased wavelength of theblue laser beam. For example, when a wavelength(λ) of the blue laserbeam is 410 nm; the numerical aperture(NA) of an objective lens is 0.6;and a thickness(t) of a disc substrate is 0.6 mm, a recording capacityof the HD-DVD is reduced to approximately 8 to 9 GB.

As described above, it is difficult for an optical recording medium tohave a recording capacity of 15 GB when using the blue laser. In orderto solve this problem, there has been attempted a scheme of reducing atrack pitch or a pit length. Various new control techniques have beenapplied so as to realize such a scheme. An example of the new controltechnique is described in a paper entitled “The path from DVD(red) toDVD(blue)”(JOINT MORIS/ISOM '97 Conference Proceeding pp. 52 to 53). Inthe paper, there is disclosed a scheme of carrying out a dynamic servoin accordance with a radial tilt angle to correct aberration and toraise recording density. However, if a tilt angle of a disc is generatedin a radial direction, the same extent of tilt angle is generated in atangential direction. Particularly, in the case of a disc having aserious surface vibration, the radial and tangential tilt angles becomelarger. Accordingly, an aberration correction through a tilt in thetangential direction is not made by carrying out only a dynamic servo ina radial direction.

An alternative technique for enlarging the recording density of anoptical recording medium is described in a paper entitled “A rewritableoptical disk system over 10 GB of capacity”(Optical Data Storage '98Conference Edition pp. 131-133). This paper suggests a scheme ofenlarging a recording density by raising the numerical aperture(NA) ofan objective lens. As described in this paper, as the numericalaperture(NA) of an objective lens increases, a tilt margin of discbecomes enlarged. Also, in order to assure the tilt margin of the disc,a thickness(t) of the disc substrate must be reduced. For example, whenthe numerical aperture(NA) is set to 0.85, the disc substrate has athickness of 0.13 mm in which a comma aberration amount(t(NA)³/λ)becomes approximately 1 so as to secure the tilt margin of the disc.According to the above-mentioned paper in which a thickness(t) of thedisc substrate is set to 0.1 mm and the numerical aperture(NA) of anobjective lens is set to 0.85, a HD-DVD capable of assuring 5.04 timesthe recording capacity of the existent DVD is provided. Moreover, thepaper considers a reduced value caused by a comma aberration margin, adefocusing aberration margin and a spherical aberration margin,including a margin of substrate thickness, etc., and the paper states aHD-DVD having a recording capacity of about 20 Gbytes can be provided.However, the scheme disclosed in the paper has a problem in that thesubstrate thickness becomes very thin, about 0.1 mm. Further, a discsurface is weakened by dust and scratches, etc. due to this thinsubstrate. Also, an objective lens having the numerical aperture of 0.85must not only combine two lenses, due to a difficulty in itsmanufacture, but also requires too short a working distance from thelens to the disc surface. It is further noted that a HD-DVD having asubstrate thickness of 0.1 mm is required to configure a blue laserpickup capable of changeably accessing the existent DVD of 0.6 mm aswell as a CD of 1.2 mm.

SUMMARY OF THE INVENTION

Accordingly, it is an object of the present invention to provide a highdensity optical recording medium that is capable of being drivenchangeably with the existent optical disc with the same drivingapparatus.

Further object of the present invention is to provide an opticalrecording/reproducing apparatus that is adapted to access compatiblywith the existent optical recording medium and the high density opticalrecording medium.

Still further object of the present invention is to provide an opticalrecording/reproducing apparatus that is adapted to access compatiblywith the high density optical recording medium and the existent opticalrecording medium using the same driving apparatus.

In order to achieve these and other objects of the invention, an opticalrecording medium according to an aspect of the present inventionincludes at least one substrate having a thickness between 0.2˜0.4 mmand at least one recording surface. The optical recording medium issuitable for recording/reproducing information by irradiating a laserbeam at a wavelength between 395˜425 nm onto the recording surface ofthe optical recording medium. The laser beam is incident on thesubstrate side of the optical recording medium through an objective lenshaving a numerical aperture of 0.62˜0.68.

An optical recording medium according to another aspect of the presentinvention includes at least one substrate and at least one recordingsurface. The optical recording medium is suitable forrecording/reproducing information by irradiating a laser beam at awavelength between 395˜425 nm onto the recording surface of the opticalrecording medium. The laser beam is incident on the substrate side ofthe optical recording medium through an objective lens. The substrateand objective lens have respectively a thickness and a numericalaperture derived form an equation as follows,

${{0.07(\lambda)} \leq {WFECrms}} = {\frac{t}{2}\frac{( {n^{2} - 1} )\mspace{11mu}\sin\mspace{11mu}\theta\mspace{11mu}\cos\mspace{11mu}\theta}{( {n^{2} - {\sin^{2}\;\theta}} )^{\frac{3}{2}}}({NA})^{3} \times \frac{1}{6\sqrt{2}}}$Wherein, “θ” and “t” are a tilting angle and thickness of the opticalrecording medium and “NA” represents the numerical aperture of theobjective lens.

An optical recording/reproducing method according to still anotheraspect of the present invention conducts recording/reproducing ofinformation by irradiating a laser beam at a wavelength between 395˜425nm onto a recording surface of an optical recording medium. The opticalrecording medium consists of at least one substrate and at least onerecording surface. The substrate has a thickness of 0.2˜0.4 mm. Thelaser beam is incident on the substrate of the optical recording mediumthrough the objective lens having a numerical aperture of 0.62˜0.68.

An optical recording/reproducing apparatus according to still anotheraspect of the present invention includes at least one laser beam sourceirradiating the laser beam at a wavelength between 396˜425 nm onto arecording surface of an optical recording medium and an objective lensfor focusing the laser beam onto the optical recording medium. Theoptical recording/reproducing apparatus conducts recording/reproducingfor information by irradiating the laser beam onto the recording surfaceof the optical recording medium. The optical recording medium has atleast one substrate and at least one recording surface. The substratehas a thickness of 0.2˜0.4 mm and the objective lens has a numericalaperture of 0.62˜0.68.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects of the invention will be apparent from thefollowing detailed description of the embodiments of the presentinvention with reference to the accompanying drawings, in which:

FIG. 1 is a characteristic diagram showing a relationship of a sphericalaberration to a disc substrate thickness;

FIG. 2 is a characteristic diagram showing a relationship of thenumerical aperture to a substrate thickness of disc when commaaberrations due to a disc tilt are 0.07λ and 0.05λ.

FIG. 3 is a sectional view showing the structure of an optical discaccording to an embodiment of the present invention;

FIG. 4 is a sectional view showing the structure of an optical discaccording to another embodiment of the present invention;

FIG. 5 is a sectional view showing the structure of an optical discaccording to still another embodiment of the present invention;

FIG. 6 is a schematic view showing the configuration of an opticalrecording/reproducing apparatus according to an embodiment of thepresent invention; and

FIG. 7 is a detailed plan view of the polarizing plate shown in FIG. 6.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Prior to describing embodiments of the present invention, a substratethickness of a HD-DVD according to the present invention allowing a bluelaser optical pickup to changeably access both the existent DVD and theHD-DVD will be contemplated. In order to allow a blue laser opticalpickup to access the HD-DVD as well as the existent DVD, an averageoptical aberration amount must be smaller than the Marchel's Criterionvalue of 0.07λ. To this end, an additional spherical aberration amountused for a calculation of the average optical aberration amount must beless than 0.07. The additional spherical aberration amount is generatedby a substrate thickness difference between the DVD and the HD-DVD.Assuming that a substrate thickness difference between the existent DVDand a HD-DVD according to the present invention is Δt, an additionalspherical aberration amount(Wspr(r)) produced at a certain position onthe objective lens by the substrate thickness difference (t is given bythe following formula:

$\begin{matrix}{{{Wsprs}(r)} = {\frac{1}{8}\frac{\frac{n^{2} - 1}{n^{3}}\Delta\;{t({NA})}^{4}}{\lambda}r^{4}}} & (1)\end{matrix}$wherein “n” represents a refraction index; “NA” is the numericalaperture of the objective lens; and “r” is a distance from the center ofthe objective lens to a certain position at which a light passes. The“r” is a value normalized by the numerical aperture(NA) of the objectivelens. A position where r=1 corresponds to a radius of a circle limitedby the clear aperture. The additional spherical aberration(Wspr(r)) atthe certain position is normalized by a wavelength(λ). If a substratethickness of a HD-DVD designed for an original objective lens is “t” anda substrate thickness difference between the DVD and the HD-DVD is “(t”,then a spherical aberration emerges even when an additional sphericalaberration Wspr(r) at a certain position on the objective lens isminimized by a focusing adjustment of the objective lens. Such aspherical aberration is called “a residual spherical aberration at acertain position on the objective lens”, which is calculated by thefollowing formula:Wspr _(rem() r)=ar ²−Wspr(r)  (2)

Subsequently, residual spherical aberrations (Wspr_rem(r)) at eachposition of the objective lens given by the above formula (2) areintegrated to thereby calculate an average value of the residualspherical aberration. Also, a square value of the residual sphericalaberration is calculated by a square operation of the residual sphericalaberrations(Wspr_rem(r)) at each position on the objective lens. Aspherical aberration(Wrms) is given by making a root mean squareoperation of the average values of the residual spherical aberrationsand the square values of each residual spherical aberration. An equationof the spherical aberration(Wrms) is expressed as follows:Wrms=√{square root over ( Wspr _(rem) ² − (Wspr _(rem)) ²)}  (3)

A constant α allowing the spherical aberration(Wrms) to be minimized canbe calculated from the given spherical aberration(Wrms). A minimumspherical aberration (WFERrms) is obtained by carrying out an operationin the equation (3) depending on this constant α. An equation of theminimum spherical aberration(WFERrms) can be expressed as follows:

$\begin{matrix}{{WFERrms} = {\frac{1}{6\sqrt{5}} \times \frac{1}{8} \times \frac{n^{2} - 1}{n^{3}}({NA})^{4}\;\Delta\; t}} & (4)\end{matrix}$

Because the conventional substrate thickness is 0.6 mm, the minimumspherical aberration(WFERrms) is changed as shown in FIG. 1 inaccordance with a substrate thickness(t) of the HD-DVD. Referring now toFIG. 1, the minimum spherical aberration(WEFRrms) is reduced graduallyas a substrate thickness(t) of the HD-DVD increases and becomes “0” whena substrate thickness(t) of the HD-DVD is equal to that of theconventional DVD. The substrate thickness(t) of the HD-DVD must be morethan 0.2 mm because such a minimum spherical aberration(WEFRrms) must beset to less than the Marchel's criterion value of 0.07λ.

Next, a substrate thickness of the HD-DVD in consideration of a tiltaffect of the disc will be described. Generally, a disc tilt occurs whena disc is accessed. Various aberrations emerge due to disc tilt. A commaaberration in these aberrations makes the largest affect on an access ofthe disc. The minimum comma aberration(WFECrms) is given by thefollowing equation: 23

$\begin{matrix}{{WFECrms} = {\frac{t}{2}\frac{( {n^{2} - 1} )\mspace{11mu}\sin\mspace{11mu}\theta\mspace{11mu}\cos\mspace{11mu}\theta}{( {n^{2} - {\sin^{2}\mspace{11mu}\theta}} )^{\frac{3}{2}}}({NA})^{3} \times \frac{1}{6\sqrt{2}}}} & (5)\end{matrix}$wherein “θ” represents a tilt angle of the disc. When the blue laseroptical pickup has the same tilt margin (θ=±0.6°) as the conventionalDVD optical pickup and, simultaneously, has a minimum comma aberrationof 0.05λ, a substrate thickness(t) of the HD-DVD is reduced like thefirst curve(CL2) in FIG. 2 as the numerical aperture of the objectivelens increases. Otherwise, when the blue laser optical pickup has thesame tilt margin (θ=±0.6°) as the conventional DVD optical pickup and,simultaneously, has a minimum comma aberration of 0.05λ, a substratethickness(t) of the HD-DVD is reduced like the second curve(CL2) in FIG.2 as the numerical aperture of the objective lens increases. FIG. 2shows that a substrate thickness of the HD-DVD must be set to below 0.45mm assuming that the numerical aperture of the objective lens is limitedto less than 0.68. In other words, when the numerical aperture of theobjective lens is limited to less than 0.68, FIG. 1 and FIG. 2 indicatesthat the substrate thickness(t) of the HD-DVD must be set to a rangelarger than 0.2 mm and smaller than 0.45 mm(i.e., 0.2<t<0.45). However,a real quality of a light beam is influenced by a substrate thicknessdifference between the discs and aberrations due to disc tilt as well asvarious aberrations including an aberration in the optical pickup.Considering such various aberrations, it is desirable that, in theHD-DVD, the numerical aperture(NA) of the objective lens is set to about0.65 and a substrate thickness(t) be set to about 0.3 mm.

Next, the numerical aperture(NA) suitable for a HD-DVD according to thepresent invention will be described in consideration of a tilt margin ofthe disc. When the minimum comma aberration(WFECrms) given by the aboveequation (5) is limited to less than 0.07λ and, at the same time, thenumerical aperture of the objective lens for a HD-DVD is limited to avalue larger than that for a conventional DVD and smaller than 0.68, asubstrate thickness(t) of the HD-DVD and the numerical aperture(NA) ofthe objective lens can be set to values within an oblique-line area inFIG. 2. It can be seen from FIG. 2 that a substrate thickness(t) of theHD-DVD according to the present invention is set to a range from 0.2 mmto 0.45 mm, and that the numerical aperture(NA) of the objective lens isset to a range from 0.62 to 0.68. Further, when a tilt margin of anoptical pickup for a HD-DVD becomes equal to a tilt margin allowed in anoptical pickup for a DVD, the substrate thickness of the disc is thinnedfrom 0.6 mm to 0.3 mm and a wavelength(λ) of beam is shortened from 650nm into 400 nm. Accordingly, the numerical aperture of the objectivelens for a HD-DVD can be increased to NA=1.085 times the numericalaperture of the objective lens for a DVD. Since the numerical apertureof the objective lens for a DVD is 0.6, the numerical aperture of theobjective lens for a HD-DVD can be set to about NA=0.6; ×1.085=0.648.

Furthermore, when intending to changeably access both a HD-DVD and a DVDusing a GaN group blue lager, that is, a light beam with a wavelength of400 nm, an effective numerical aperture adaptive for the DVD will betaken into consideration. Assuming that a wavelength of a beam generatedfrom the blue laser is λ=400 nm and a refraction index of the substrateis n=1.5, an effective numerical aperture of the objective lens, inwhich the minimum spherical aberration(WFErms) is 0.05, will beobtained. At this time, the size of a beam spot concentrated on therecording face of the disc must be considered along with the sphericalaberration amount. The size of the beam spot concentrated on therecording face becomes proportional to λ/NA. Since a wavelength(λ) ofbeam applied to a DVD is 650 nm and the numerical aperture(NA) of theobjective lens is 0.6, the first effective numerical aperture Naeff1 ofthe objective lens for a HD-DVD becomes (0.6/650)×400=0.369.Accordingly, when a DVD is reproduced using the blue laser, a DVD with athickness of 0.6 mm is accessible, too, if the numerical aperture of theobjective lens is controlled such that the first effective numericalaperture(Naeff1) of the objective lens becomes 0.369. Also, when it isintended to obtain a beam spot size for accessing a CD using the bluelaser (λ=410 nm), since a wavelength(λ)of beam applied to a CD is 780 nmand the numerical aperture(NA) of the objective lens is 0.45, aneffective numerical aperture(Naeff2) of the objective lens for a HD-DVDbecomes 0.45×(400/780)=0.231. Accordingly, if the blue laser is used andthe numerical aperture of the objective lens is controlled such that aneffective numerical aperture(Naeff2) of the objective lens becomes0.231, a CD with a thickness of 1.2 mm can be accessed by the blue laseroptical pickup. For example, a control of the numerical aperture of theobjective lens can be achieved by making use of a numerical aperturecontrol device and a twin objective lens.

Preferred embodiments of the present invention will be described indetail with reference to FIG. 3 to FIG. 7. FIG. 3 is a sectional viewshowing the structure of a HD-DVD according to an embodiment of thepresent invention. The HD-DVD includes a first substrate 10 having a pitpattern defined on its surface, a reflective film 12 grown on the firstsubstrate 10, and a second substrate 14 having a pit pattern opposed tothe pit pattern defined on the first substrate 10 on its lower surface.The first substrate 10 is made from a transmissive material such aspolycarbonate, etc. The pit pattern provided on the surface of the firstsubstrate 10 is formed by an inversive transcription method. Also, thepit pattern on the first substrate 10 includes audio, text and imageinformation, etc. In other words, the surface of the first substrate 10provided with the pit pattern is used as a recording film. Thereflective film 12 formed on the surface of the first substrate 10reflects a light beam received by way of the second substrate 14. Thesecond substrate 14 is bonded to the first substrate 10 in such a mannerthat the lower surface thereof provided with the pit pattern contactsthe reflective film 12. Also, the second substrate 14 is made from atransmissive material such as polycarbonate, etc. like the firstsubstrate 10. Such a second substrate 14 is used as a light transmissivelayer. The second substrate 14 has a thickness of 0.3 mm as describedabove so that it is changeably used for the existent CD and DVD. Thesurface of the second substrate 14, which is a light transmissive layerin contact with the reflective film 12, becomes a substantial recordingface. On the other hand, the first substrate 10 plays a role as aprotective film for preventing a deterioration of the reflective film12. To this end, the first substrate 10 is formed more thickly than thesecond substrate 14, and has preferably a thickness of 0.9 mm in such amanner that an entire thickness of the HD-DVD becomes 1.2 mm.

FIG. 4 shows the structure of a HD-DVD according to another embodimentof the present invention. The HD-DVD further includes a third substratebonded to the lower portion of the first substrate 10 in comparison tothe HD-DVD of FIG. 3. The third substrate 16 is responsible forpreventing a bending deformation, etc. of the disc due to a change in adriving environment such as humidity, etc. It is desirable that thethird substrate 16 is formed to a thickness of 0.3 mm equal to thesecond substrate 14 in consideration of symmetry with the secondsubstrate 14. In this embodiment, the first substrate 10 is formed in athickness of 0.6 mm such that the entire thickness of the HD-DVD becomes1.2 mm in accordance with an addition of the third substrate 16.

FIG. 5 shows the structure of a HD-DVD according to still anotherembodiment of the present invention. Referring to FIG. 5, the HD-DVDincludes a first substrate 18 having a pit pattern formed on each of theupper surface and the lower surface thereof, and first and secondreflective films 20A and 20B grown on the upper surface and the lowersurface thereof, respectively. A second substrate 20A is bonded on thefirst reflective film 22A while a third substrate 22B is bonded on thelower surface of the second reflective film 20B. The second substrate22A is used as a light transmissive layer with respect to the firstreflective film 20A. The second substrate 22A has a pit pattern providedat its lower surface bonded to the first reflective film 20A. The pitpattern defined on the lower surface of the second substrate 22A has ashape engagable to the pit pattern defined on the surface of the firstsubstrate 18. Similarly, the third substrate 22B is used as a lighttransmissive layer with respect to the second reflective film 20B. Also,the third substrate 22B has a pit pattern provided on the surface bondedto the second reflective film 20B. The pit pattern defined on thesurface of the third substrates 22A has a shape engagable to the pitpattern defined on the lower surface of the first substrate 18. Each ofthe second and third substrate 22A and 22B have a thickness of 0.3 mmbecause they are used as light transmissive layers. Accordingly, thefirst substrate 18 has a thickness of 0.6 mm such that the entirethickness of the HD-DVD becomes 1.2 mm.

It should be understood by an ordinary skilled person in the art that,although the embodiments of the present invention disclosed in FIG. 3 toFIG. 5 are limited to a disc of reproduction-only type, the presentinvention is applicable to a recordable disc. For instance, HD-DDSaccording to embodiments of the present invention shown in FIG. 3 toFIG. 5 further includes a recording material layer on the bonded surfaceamong the reflective films 12 and 20 and the light transmissive layersso that it can be used as a recordable disc.

Referring now to FIG. 6, there is schematically shown a lightrecording/reproducing apparatus according to an embodiment of thepresent invention. The light recording/reproducing apparatus includes ablue laser 32 for irradiating a light beam onto a HD-DVD 30A, a DVD 30Bor a CD 30C, and an objective lens part 42 for concentrating a lightbeam on the recording face of the HD-DVD 30A, the DVD 30B or the CD 30C.A liquid crystal panel 34, a collimator lens 36, a beam splitter 38 anda polarizing plate 40 are sequentially arranged between the blue laser32 and an objective lens part 42. Also, the light recording/reproducingapparatus further includes a photo detector 46 for converting a lightbeam reflected by the recording face of the HD-DVD 30A, the DVD 30B orthe CD 30C into an electrical signal, and a sensor lens 44 positionedbetween the beam splitter 38 and the photo detector 46. The HD-DVD 30A,the DVD 30B and the CD 30C have light transmissive layers with thicknessof 0.3 mm, 0.6 mm and 1.2 mm, respectively. The blue laser 32 generatesa light beam with a wavelength of 400 nm. A light beam generated fromthe blue laser 32 may have characteristics of a vertical linepolarization, a horizontal line polarization and a circle polarization,etc., but it is assumed to have the vertical line polarizationcharacteristic for the sake of convenience. In other words, it isassumed that a light beam generated from the blue laser 32 is a verticalline polarizing beam. The collimator lens 36 converts a divergent lightbeam progressing from the blue laser 32, received via the liquid crystalpanel 34, into a parallel light beam to thereby prevent a leakage of thelight beam. The beam splitter 38 allows a light beam from the collimatorlens 36 to progress, via the polarizing plate 40, toward the objectivelens part 42, and, simultaneously, allows a reflective light beamreflected from the recording face of the HD-DVD 30A, the DVD 30B or theCD 30C to be received via the objective lens part 42, the polarizingplate 40, the sensor lens 44 by the photo detector 46. The sensor lens44 concentrates a light beam progressing from the beam splitter 38 tothe photo detector 46 on the surface of the photo detector 46, therebypreventing a leakage of a light beam. The photo detector 46 converts areflective light beam reflected by the HD-DVD 30A, the DVD 30B or the CD30 and then received via the objective lens part 42, the polarizingplate 40, the beam splitter 38 and the sensor lens 44 into an electricalsignal. The electrical signal usually includes a servo signal and aninformation signal.

The objective lens part 42 includes first and second objective lenses 42and 42B having numerical apertures (NA) different from each other. Thefirst and second objective lenses 42A and 42B are installed at a singlesupporting member and selectively located on a light path by rotatingthe supporting member depending upon a type of optical disc. Thesupporting member is rotated with an actuator(not shown), and theactuator usually drives the supporting member in an axis sliding systemin such a manner that the supporting member is pivoted around a rotationaxis. The first objective lens 42 has the first numerical aperture(NA1),i.e., 0.65 while the second objective lens 42B has the second numericalaperture(NA2), i.e., 0.369. Also, the second numerical aperture(NA2) canbe controlled into the third numerical aperture(NA3)(i.e., 0.231) modeby means of a numerical aperture control device as described later.

The liquid crystal panel 34 arranged between a light source 32 and thecollimator lens 36 and the polarizing plate 40 arranged between the beamsplitter 38 and the objective lens part 42 control the numericalaperture of the second objective lens 42B. The liquid crystal panel 34is responsible for changing a polarization characteristic of a lightbeam depending on whether or not a voltage is applied, and selectivelyshuts off a portion of the light beam depending on a polarizationcharacteristic of an incident beam. More specifically, the liquidcrystal panel 34 selectively rotates a vertical polarized beamprogressing from the light source 32 to the collimator lens 36 at 90°(depending on whether or not a voltage is applied. For example, when theHD-DVD 30A or the DVD 30B is accessed, a high voltage is applied to theliquid crystal panel 34. At this time, the liquid crystal panel 34allows a vertical line polarized light beam from the blue laser 32 to bepassed in a state of keeping an original polarization characteristic. Onthe other hand, when the CD 30C is accessed, a low voltage is applied tothe liquid crystal panel 34. Then, the liquid crystal panel 34 rotates avertical line polarized beam from the blue laser 32 at 90° and convertsthe same into a horizontal polarized beam. Otherwise, when a horizontalline polarized beam is generated from the blue laser 32, the liquidcrystal panel 34 selectively changes a polarization characteristic ofthe horizontal polarized beam in accordance with a voltage contrary tothe above-mentioned voltage. As shown in FIG. 7, the polarizing plate 40includes a circular non-polarizing area 40A, and a polarizing area 40Bdefined around the non-polarized area 40A. The non-polarizing area 40Apositioned at the center of such a polarizing plate 40 passes anincident light beam toward the objective lens 42 independently of apolarization characteristic of the incident light beam. The polarizingarea 40B passes an incident light beam when a polarized direction of thelight beam is identical to its polarized direction, whereas it shuts offan incident light beam when a polarized direction of the light beam isdifferent from its polarized direction. In other words, when an incidentlight beam is a vertical polarized beam, that is, when the HD-DVD 30A orthe DVD 30B is accessed, the polarizing area 40B passes the verticalline polarized light beam toward the objective lens part 42 as it islike the non-polarizing area 40A. On the other hand, when an incidentlight beam is a horizontal line polarized light beam, that is, when theCD 30C is accessed, the polarizing area 40B shuts off the horizontalline polarized light beam in a contrary manner to the non-polarizingarea 40A. In this case, the non-polarizing area 40A makes the secondobjective lens 42B have the third numerical aperture(NA3). Also, thenon-polarizing area allows the second objective lens 42B with the secondnumerical aperture(NA2) of 0.369 to be changed into the third numericalaperture(NA3) of 0.231.

Subsequently, a case of accessing the HD-DVD 30A, the DVD 30B or the CD30C will be described in detail. First, when the HD-DVD is accessed, thefirst objective lens 42 is located at a light path and a high voltage isapplied to the liquid crystal panel 34. Accordingly, a vertical linepolarized beam generated from the blue laser 32 is incident to the firstobjective lens 42 via the liquid crystal panel 34, the collimator lens36, the beam splitter 38 and the polarizing plate 40. This incidentlight beam is converged by means of the first objective lens 42 with thefirst numerical aperture(NA1) to thereby be irradiated onto therecording face of the HD-DVD 30A in a spot shape with a size suitablefor the HD-DVD 30A. Next, when the DVD 30B is accessed, a high voltageis applied to the liquid crystal panel 34 and, at the same time, thesecond objective lens 42B with the second numerical aperture(NA2) islocated in the light path. At this time, the second objective lens 42Bis arranged in the light path by pivoting the supporting member of theobjective lens part 42 with an actuator. Accordingly, a vertical linepolarized light beam generated from the blue laser 32 is incident to thesecond objective lens 42B via the liquid crystal panel 34, thecollimator lens 36, the beam splitter 38 and the polarizing plate 40.Then, the second objective lens 42B having the second numericalaperture(NA2) focuses a light beam to thereby irradiate a light beam onthe recording face of the DVD 30B in a spot shape having a size suitablefor the DVD 30B. Finally, when the CD 30C is accessed, a low voltage isapplied to the liquid crystal panel 34 and, at the same time, the secondobjective lens 42B is located on a light path. Accordingly, a verticalline polarized beam generated from the blue laser 32 is incident to thepolarizing plate 40 via the collimator lens 36 and the beam splitter 38in a state changed into a horizontal line polarized beam by the liquidcrystal panel 34. The horizontal line polarized beam incident to thepolarizing plate 40 allows the outer portion thereof to be shut off bythe polarizing area 40B and the center portion thereof to go through thenon-polarizing area 40A, thereby having a reduced flux diameter. Thevertical line polarized beam with the reduced flux diameter allows thesecond objective lens 42B to have the third numerical aperture(NA3) withan area corresponding to the non-polarizing area 40A. Accordingly, thesecond objective lens 42B allows a light beam to be irradiated onto therecording face of the CD 30C in a spot shape having a size suitable forthe CD 30C.

As a result, a numerical value of a wavelength (λ) of a light beamcorresponding to the HD-DVD 30A, the DVD 30B or the CD 30C and thenumerical aperture(NA) of the objective lens can be given as indicatedin the following table:

TABLE 1 WAVELENGTH (λ) NUMERICAL APERTURE HD-DVD 400 nm 0.65 DVD 400 nm0.369 CD 400 nm 0.231

A recording/reproducing operation can be carried out compatibly withthree optical disc even when a device is configured by combining variousnumerical aperture control means besides the embodiment shown in FIG. 6.For instance, Korean Patent Application Nos. 98-11972, 98-11973 and98-11974 filed by the present applicant has suggested examples of thenumerical aperture control means. Alternatively, if a liquid crystalpanel is combined with an annular mark, then it is possible to providethree numerical aperture modes using a single objective lens. A systememploying this mark is disclosed in a paper entitled “a compact disccompatible digital video disc pickup using annular mask”(“Joint ISOM/ODS'96 Vol. 12, pp. 348-350”).

As described above, the optical recording/reproducing apparatus makesuse of a blue laser as a light source and is capable of controlling thenumerical aperture of the objective lens into three modes, so that itcan changeably record and reproduce a HD-DVD according to the presentinvention as well as the existent CD and DVD. Further, it should benoted that, although an optical recording/reproducing apparatusapplicable changeably to all the three optical discs of HD-DVD, DVD andCD has been described as an example in the above embodiments, it ispossible to construct an optical recording/reproducing apparatusapplicable changeably to only two optical discs, i.e., HD-DVD and DVD;or HD-DVD and CD, etc. Meanwhile, the above-mentioned HD-DVD can assureonly a capacity of 13.8 GB. Accordingly, if it is intended to extendsaid 13.8 GB capacity into 15 GB, then an additional capacity of 8% isrequired. This can be easily attained by applying a technique ofeliminating a cross talk between the adjacent tracks after narrowing thetrack pitch by 8% or a technique of reducing a tangential ISI.

As described above, a HD-DVD according to the present invention caneasily obtain the required large capacity by making use of an objectivelens having a substrate thickness of 0.2 to 0.45 and the numericalaperture of 0.62 to 0.68 in correspondence with the blue laser. Further,the optical recording/reproducing method and apparatus according to thepresent invention controls the numerical aperture of the objective lensinto 0.369 to 0.231 in accordance with the CD and DVD, so that it canperform a recording and reproducing operation changeably with theexistent CD and DVD as well as the HD-DVD according to the presentinvention.

It should be understood to the ordinary skilled person in the art that,although a standard value of the numerical aperture corresponding to theHD-DVD, the DVD or the CD has been suggested in the present invention,an object of the present invention can be achieved even though thenumerical aperture varies in a range of about 10%.

Although the present invention has been explained by the embodimentsshown in the drawings described above, it should be understood to theordinary skilled person in the art that the invention is not limited tothe embodiments, but rather that various changes or modificationsthereof are possible without departing from the spirit of the invention.Accordingly, the scope of the invention shall be determined only by theappended claims and their equivalents.

1. An optical recording medium suitable for recording or reproducinginformation by irradiating a laser beam at a wavelength between 395˜425nm onto a recording surface in the optical recording medium, the laserbeam being incident from a side of the optical recording medium throughan objective lens having a numerical aperture of 0.62˜0.68, the opticalrecording medium comprising: at least first and second substrates, areflective film formed between the first and second substrates, and atleast one recording surface, the first and second substratesrespectively having a thickness greater than 0.2 mm, wherein a totalthickness of the optical recording medium is substantially 1.2 mm and acapacity of said recording medium is greater than 13.8 Gbytes per onerecording surface, said laser beam being irradiated onto the recordingsurface with said wavelength between 395˜425 nm through said objectivelens having said numerical aperture of 0.62˜0.68.
 2. The optical mediumof claim 1, the second substrate is formed over the first substrate, andhas a pit pattern on a surface thereof facing the first substrate. 3.The optical medium of claim 1 further comprising a third substrate,wherein the second substrate is formed over a first surface of the firstsubstrate, and the third substrate is formed over a second surface,which is the opposite surface of the first surface, of the firstsubstrate.
 4. The optical medium of claim 3, wherein the third substratehas a same thickness as the second substrate.
 5. The optical medium ofclaim 3, wherein the second substrate has a first pit pattern, and thethird substrate has a second pit pattern.
 6. The optical medium of claim3, Wherein the first substrate has a first pit pattern on the firstsurface thereof and a second pit pattern on the second surface thereof.7. The optical medium of claim 3, further comprising a second reflectivefilm formed between the first and third substrates.
 8. The opticalrecording medium as claimed in claim 1, wherein an optical aberrationdepending on a thickness of the substrate, a tilt margin, the wavelength and the numerical aperture is less than 0.07 λ, where the λ isthe wave length.
 9. An optical recording or reproducing method ofconducting recording or reproducing of information, comprising:irradiating a laser beam at wavelength between 395˜425 nm onto anoptical recording medium which has at least first and second substrates,a reflective film formed between the first and second substrates and atleast one recording surface, wherein the first and second substratesrespectively have a thickness of more than 0.2 mm and a total thicknessof the optical recording medium is about 1.2 mm, and a capacity of theoptical recording medium is greater than 13.8 Gigabytes per onerecording surface, and wherein the laser beam being incident on thesubstrate of the optical recording medium using an objective lens havinga numerical aperture of 0.62˜0.68.
 10. An optical recording orreproducing apparatus for conducting recording or reproducing ofinformation, the apparatus comprising: at least one laser beam sourceirradiating the laser beam at a wavelength between 395˜425 nm onto anoptical recording medium which has at least first and second substrates,a reflective film formed between the first and second substrates and atleast one recording surface, wherein the first and second substratesrespectively have a thickness of more than 0.2 mm and total thickness ofthe optical recording medium is about 1.2 mm, and a capacity of theoptical recording medium is greater than 13.8 Gigabytes per onerecording surface; and an objective lens for focusing the laser beamonto the optical recording medium, the objective lens having a numericalaperture of 0.62˜0.68.
 11. The optical recording or reproducingapparatus as claimed in claim 10, further comprising: a numericalaperture control device controlling the numerical aperture of theobjective lens into 0.35 to 0.40, thereby recording or reproducing arecording medium with a substrate thickness of approximately 0.6 mm. 12.The optical recording or reproducing apparatus as claimed in claim 10,wherein the numerical aperture control device controls the numericalaperture of the objective lens into about 0.24, thereby recording orreproducing a recording medium with a substrate thickness ofapproximately 1.2 mm.
 13. The optical recording or reproducing apparatusas claimed in claim 10, further comprising: a numerical aperture controldevice controlling the numerical aperture of the objective lens into anyone of 0.35 to 0.40 and about 0.24, thereby selectively recording orreproducing a recording medium with a substrate thickness ofapproximately 0.6 mm and a recording medium with a substrate thicknessof approximately 1.2 mm.
 14. An optical recording medium suitable forrecording or reproducing information by irradiating a laser beam at awavelength between 395˜425 nm onto at least one recording surface in theoptical recording medium, the laser beam being incident via at least onesubstrate of the optical recording medium by an objective lens having anumerical aperture of 0.62˜0.68, the substrate having a thicknessgreater than 0.2 mm and the optical recording medium having a capacitybeing more than 13.8 Gbytes per recording surface, the recording mediumcomprising: a first substrate; a second substrate formed over a surfaceof the first substrate; the second substrate having a pit patter on asurface facing the first substrate;and a reflective film formed betweenthe first and second substrates.
 15. An optical recording mediumsuitable for recording or reproducing information by irradiating a laserbeam at a wavelength between 395˜425 nm onto at least one recordingsurface in the optical recording medium, the laser beam being incidentvia at least one substrate of the optical recording medium by anobjective lens having a numerical aperture of 0.62˜0.68, the substratehaving a thickness greater than 0.2 mm,and the optical recording mediumhaving a capacity being greater than 13.8 Gbytes per recording surface,the optical recording medium comprising: a first substrate; a secondsubstrate formed over a first surface of the first substrate; and athird substrate formed over a second surface, which is the oppositesurface of the first surface, of the first substrate
 16. The opticalmedium of claim 15, wherein the third substrate has a same thickness asthe second substrate.
 17. The optical medium of claim 19 wherein thesecond substrate has a first pit pattern, and the third substrate has asecond pit pattern.
 18. The optical medium of claim 15, wherein thefirst substrate has a first pit pattern on the first surface thereof anda second pit pattern on the second surface thereof,
 19. The opticalmedium of claim 15, further comprising: a first reflective film formedbetween the first and second substrates; and a second reflective filmformed between the first and third substrates.
 20. The optical medium ofclaim 19, wherein a total thickness of the first substrate, the firstreflective film, the second substrate, the second reflective film, andthe third substrate substantially equals 1.2 mm.
 21. An opticalrecording or reproducing apparatus for conducting recording/reproducingfor information, the apparatus comprising: at least one laser beamsource irradiating the laser beam at a wavelength between 395˜425 nmonto an optical recording medium which has at least first and secondsubstrates, a reflective film formed between the first and secondsubstrates and at least one recording surface, wherein the first andsecond substrates respectively have a thickness of more than 0.2 mm anda capacity of the optical recording medium is greater than 13.8Gigabytes per one recording surface; an objective lens focusing thelaser beam onto the optical recording medium, the objective lens havinga numerical aperture of 0.620˜0.68; and a numerical aperture controldevice controlling the numerical aperture of the objective lens into0.35 to 0.40, thereby recording or reproducing a recording medium with asubstrate thickness of approximately 0.6 mm.
 22. The optical recordingor reproducing apparatus as claimed in claim 21, wherein the numericalaperture control device controls the numerical aperture of the objectivelens into about 0.24, thereby recording or reproducing a recordingmedium with a substrate thickness of approximately 1.2 mm.
 23. A methodfor recording/reproducing information on/from an optical recordingmedium having at least one substrate and one recording layer, the methodcomprising: determining a thickness of the substrate in the opticalrecording layer: changing a numerical aperture of a objective lensaccording to the determined thickness; and irradiating a laser beam atwavelength between 395˜425 run by using an objective lens of the changednumerical aperture onto the optical recording medium, wherein an opticalaberration which depends on at least one from the thickness of thesubstrate, the tilt margin, the wavelength and the numerical aperture,is less than 0.07 λ, where the λ is the wavelength, and wherein thenumerical aperture of the objective lens is changed into about 0.24, ifthe thickness of the substrate is about 1.2 mm.
 24. An apparatus forrecordin/reproducing information on/from an optical recording mediumhaving at least one substrate and one recording layer, the apparatuscomprising: a laser beam source irradiating the laser beam at awavelength 395˜425 nm onto an optical recording medium; an objectivelens for focusing the laser beam onto the optical recording medium; anda numerical aperture control device for controlling the numericalaperture of the objective lens according to a thickness of thesubstrate, wherein an optical aberration which depends on at least onefrom the thickness of the substrate, the tilt margin, the wavelength andthe numerical aperture, is less than 0.07 λ, where the λ is thewavelength, wherein the numerical aperture control device changes thenumerical aperture of the objective lens into about 0.24, if thethickness of the substrate is about 1.2 mm.